This invention relates generally to a semiconductor switch, and more particularly to a semiconductor cross-point switch suitably usable in a telephone exchange.
For many years, the cross bar switch, which is a metallic contact switch, has been employed as the basic switch of a telephone exchange. Since the cross bar switch is a mechanical switch, the operating speed is low. Moreover, the metal of the contacts is subject to heavy wear, which reduces the lifetime and reliability of the switch. Another disadvantage is that noise which arises from the contact is great. Further, exchanges employing the cross bar type switch are large in size and heavy in weight, and therefore require a large place of installation. Such cross bar switches also suffer from the disadvantage of a high maintenance cost on account of a high driving power of the switch.
Telephone exchanges have become quite large due to a great increase in the number of circuits, and have often been used in coupling with electronic systems such as picture transmission systems and computers. Therefore, a switch for an exchange should preferably be small in size and, simultaneously, high in the operating speed and reliability. Accordingly, it is desirable to use a semiconductor cross-point switch to replace the cross bar switch since such a cross-point switch operates electronically and is high in its response rate.
Characteristics required for the switch of the telephone exchange are that the withstand voltage is high, a high current being permitted to flow, and that the resistance at turning-on is low, while the resistance at turning-off is high. In order to easily attain these characteristics, it is generally most suitable to employ a thyristor for a switching circuit. The cross-point switch which employs the thyristor for the switching circuit should include a driving circuit and a protective circuit along with the switching circuit in order to fulfill the switching function as explained hereinafter.
The cross-point switch which is fabricated by arranging the switching circuit portions, the driving circuit portions and the protective circuit portions without a specific order on a chip causes problems in that it operates erroneously. Further, since the wiring becomes complicated, the chip size is large, and the density of integration becomes low. It has been revealed that since a voltage being as high as 200 V is applied to the switching circuit portions, the switching circuit portions interfere with one another to have undesirable effects on the switching characteristics when they are arranged adjacent to one another. Ordinarily, any high voltage applied to the driving circuit portions or the protective circuit portions does not affect other portions, so that they may be arranged in adjacency. Moreover, a parasitic capacitance develops under a metallic interconnection. In the case of arranging the circuits in a disorderly manner, the interconnection becomes long. This causes the parasitic capacitance to become large, which can cause the malfunction of the switching circuit portion.
In U.S. Pat. No. 4,015,143 a semiconductor switch is described which is large in dv/dt bearing capacity regardless of the anode or cathode potential, high in breakdown voltage, capable of being closed with a small control current, and easy to be included in semiconductor integrated circuits.
West German Offenlegungsschrift No. 2,655,622 sets forth a partial plan view showing a semiconductor integrated circuit which is equipped with a thyristor device.
These references, however, do not describe a disposition of the switching circuit portions, the driving circuit portion and the protective circuit portions on a chip in a manner to prevent interference between the switching circuit portions.